This invention relates to a method and an operating circuit for a gas discharge lamp. More particularly, the invention relates to a method and a ballast circuit each of which provides a signal having a selected band of frequencies for operating a xenon-metal halide lamp that is particularly suited for vehicle applications.
U.S. patent application Ser. No. 157,436 filed Feb. 18, 1988 of R. S. Bergman et al. assigned to the same assignee as the present invention and herein incorporated by reference, discloses a xenon-metal halide lamp particularly suited for automotive applications. The xenon-metal halide lamp provides improved efficiency and longer life relative to incandescent lamps while a high pressure xenon gas within the lamp primarily achieves instant light capabilities which makes such a lamp particularly suited for vehicle applications.
U.S. patent application Ser. No. 320,736 filed Mar. 8, 1989 of G. R. Allen et al. discloses a method and a circuit for acoustically operating a xenon-metal halide lamp during its various modes within selected bands of frequencies and allowing such a lamp to be operated in a horizontal orientation that is particularly suited for vehicle applications. U.S. patent application Ser. No. 320,736 teaches a method and ballast that reduces the arc bowing typically experienced by a high pressure xenon-metal halide lamp and also allows for a high pressure xenon-metal halide lamp to serve as a source of light for both high and low beam illumination patterns for the vehicle.
The ballast of U.S. application Ser. No. 320,736 serves well the needs of the metal halide discharge lamp but has somewhat of a limitation in that it does not disclose the means for adjusting the power that may be applied to the xenon metal halide lamp for regulation purposes. For example, it is desired to reduce the power applied to the lamp by a factor of approximately 3-4 from its initial start mode of operation to its final or run mode of operation. Such a reduction should be accomplished without varying the desired operating frequency of the signal applied to the lamp so that the arc within the lamp is not bowed significantly.
Accordingly, it is an object of the present invention to provide an AC ballast circuit that allows the power applied to the lamp to be decreased from its start mode value to its desired value occurring during its run mode of operation without varying the desired operating frequency so as to regulate the power to the metal halide lamp while still yielding a non-bowed arc condition.
It is a further object of the present invention to provide a ballast circuit that provides desired signals for operating the metal halide lamp in a horizontal orientation which is particularly suited for vehicle applications.